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May 27, 2008 -- Adults with attention deficit hyperactivity disorder (ADHD) work 22 fewer workdays than their co-workers without the condition, according to a new study. The total takes into account days absent and days of low productivity.

"When you compare these people to other people of the same age, sex, and with the same education and see how they function [on the job], there is a substantial amount of role impairment in ADHD," says Ron Kessler, PhD, a co-author of the study and a professor of health care policy at Harvard Medical School.

The study was part of the World Mental Health Survey Initiative of the World Health Organization (WHO). It is published online ahead of print in Occupational and Environmental Medicine.

Kessler and his team screened more than 7,000 workers (including self-employed workers) aged 18 to 44. The workers described their performance on the job in the last month.

While ADHD is often thought of as a childhood problem, growing evidence suggests that a substantial number of adults have the condition and that it is underdiagnosed in adults. Those with ADHD find it difficult to concentrate due to their distractibility, hyperactivity, impulsivity, or forgetfulness.

Adults With ADHD: The Findings

Though the WHO World Mental Health Surveys are conducted in 26 countries, the recent study assessed only 10 countries for the prevalence of adult ADHD, Kessler says, because these 10 were the only ones who "bothered to look at ADHD."

"What we discovered is a pretty substantial number of people have ADHD," says Kessler.

Overall, "we had about 3.4% of people who had ADHD."

Country by country, France had the highest percentage of ADHD among adult workers surveyed, followed by the U.S.; Spain had the lowest percentage.

The 10-country scorecard:

France 7.3%

U.S. 5.2%

Netherlands 5.0%

Belgium 4.1%

Germany 3.1%

Italy 2.8%

Mexico and Colombia: 1.9%

Lebanon 1.8%

Spain 1.2%

Adults With ADHD: The Time Loss

Overall, Kessler says, those with ADHD, compared with workers without the condition, put in 22.1 fewer days a year -- nearly a month's worth of workdays. About eight of those days were completely lost because they didn't work or didn't carry out normal activities. The other 14 days had low productivity, where the quantity or the quality of the work suffered, Kessler says.

"People with ADHD have more sick days and lower performance when they work," Kessler tells WebMD. "This is one of those hidden illnesses in the workplace."

Adults With ADHD

The problem of ADHD in the workplace, until now, has not been investigated much, says James T. McCracken, MD, vice chairman of the department of psychiatry and biobehavioral sciences at the University of California Los Angeles Semel Institute and Resnick Neuropsychiatric Hospital.

"Overall [the study] raises a really important issue," he says. "It takes a look at an aspect of adult ADHD that until now has been not that well quantified -- the work impact."

The numbers obtained by the researchers make sense, he says, from what he has observed with patients.

"The finding that one month a year is lost is very significant from a productivity standpoint," says Thomas Parry, PhD, president of the Integrated Benefits Institute, a San Francisco-based nonprofit organization that conducts research on workplace health and productivity.

Parry is working with Kessler to develop tools to better measure ADHD in the workplace.

Advice for Adults With Possible ADHD

Workers who have trouble concentrating and focusing, Kessler says, should bring it up with their primary care doctor. "It's worth talking to your doctor because you don't have to live like this."

McCracken agrees, noting that medication can help. Behavioral therapy that addresses key problem areas such as poor organization have been shown to help, too, he says.

The WHO Mental Health Survey Initiative is supported by the U.S. National Institute of Mental Health, other public health organizations, pharmaceutical companies, and other sources. The preparation of the current report was partially supported by Eli Lilly and Co., which makes an ADHD drug, as well as other survey initiative supporters. Lilly had no role in the study design, results, or analysis.